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SubjectRe: PROBLEM: __offline_isolated_pages may offline too many pages
On Thu, Dec 16, 2010 at 09:06:57AM +0900, KAMEZAWA Hiroyuki wrote:
> On Wed, 15 Dec 2010 10:25:51 +0100
> Ingo Korb <ingo@akana.de> wrote:
>
> > On 15.12.2010 01:21, KAMEZAWA Hiroyuki wrote:
> >
> > > It's designed for offline memory section> MAX_ORDER. pageblock_nr_pages
> > > is tend to be smaller than that.
> > >
> > > Do you see the problem with _exsisting_ user interface of memory hotplug ?
> > > I think we have no control other than memory section.
> >
> > The existing, exported interface (remove_memory() - the check itself is
> > in offline_pages()) only checks if both start and end of the
> > to-be-removed block are aligned to pageblock_nr_pages. As you noted the
> > actual size and alignment requirements in __offline_isolated_pages can
> > be larger that that, so I think the checks in offline_pages() should be
> > changed (if 1<<MAX_ORDER is always >= pageblock_nr_pages) or extended
> > (if there can be any relation between the two).
> >
>
> Ok, maybe my mistake. This is a fix. Thank you for reporting.
> ==
>
> offline_pages()'s sanity check of given range is wrong. It should
> be aligned to MAX_ORDER. Current exsiting caller uses SECTION_SIZE
> alignment, so this change has no influence to exsisting callers.
>
> Reported-by: Ingo Korb <ingo@akana.de>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Other than the spelling mistakes in the changelog and the lack of a
subject;

Acked-by: Mel Gorman <mel@csn.ul.ie>

> ---
> mm/memory_hotplug.c | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> Index: linux-2.6.37-rc5/mm/memory_hotplug.c
> ===================================================================
> --- linux-2.6.37-rc5.orig/mm/memory_hotplug.c
> +++ linux-2.6.37-rc5/mm/memory_hotplug.c
> @@ -798,10 +798,14 @@ static int offline_pages(unsigned long s
> struct memory_notify arg;
>
> BUG_ON(start_pfn >= end_pfn);
> - /* at least, alignment against pageblock is necessary */
> - if (!IS_ALIGNED(start_pfn, pageblock_nr_pages))
> + /*
> + * Considering buddy allocator which joins nearby pages, the range
> + * in offline should be aligned to MAX_ORDER. If not, isolated
> + * page will be joined to other (not isolated) pages.
> + */
> + if (!IS_ALIGNED(start_pfn, MAX_ORDER_NR_PAGES))
> return -EINVAL;
> - if (!IS_ALIGNED(end_pfn, pageblock_nr_pages))
> + if (!IS_ALIGNED(end_pfn, MAX_ORDER_NR_PAGES))
> return -EINVAL;
> /* This makes hotplug much easier...and readable.
> we assume this for now. .*/
>

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab


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